A power steering system of this type is known from U.S. Pat. No. 5 553 683. This known power steering system possesses three valves, the opening state of which is controlled by an electronic control by means of pulse-width modulation. In this case, the opening and closing behaviour of the valves is controlled directly, and incorrect settings of the boosting force may occur at least temporarily.
According to the periodical O +P "Olhydraulik und Pneumatik" ["Oil Hydraulics and Pneumatics"] 40 (1996) No. 6, page 409, closed-center hydraulic power steering systems are known, a high peak power being available if a hydraulic pressure accumulator arranged as a pressure source is dimensioned appropriately. It is advantageous, furthermore, that the hydraulic steering system consumes energy only when a steering adjustment of the steerable vehicle wheels is assisted by a boosting force. By contrast, in the case of a steady-state steering situation without any manual force, no hydraulic energy is required.
In a power steering system known from EP-A 0 427 029, the control-valve arrangement is formed by proportionally working control valves which are urged by spring force into a position connecting the respective side of the servomotor to the hydraulic reservoir and by a controllable electromagnet, counter to the spring force, into a position connecting the respective side of the servomotor to the pressure accumulator.
It is known from U.S. Pat. No. 4,562,710 to provide hydraulic blocking of the servomotor of a power steering system as an immobilizer.
In power steering systems of the type specified in the introduction, a sensitive control of the boosting force may present difficulties.